Free Physics NEET Video Lectures for Dual Nature Of Matter And Radiation

00:00:00 sh [Music] [Music] [Music] dear students of Class 12 my name is V Rish Shankar I'm a physics teacher in I Delhi and what I'm going to do in the next seven or eight lectures we don't know how many is to discuss the topics on the so-call called modern physics in your 12th standard Physics course so essentially they will be chapters 11 to 13 and what we are going to cover are photoelectric effect the Deep rolly waves bore model of the atom and nuclear physics this course is qualitatively

00:01:24 different from whatever you have studied in your 11th and 12th standard you covered a whole lot of courses for example you studied mechanics both Statics and Dynamics then you studied electricity and magnetism thermodynamics Optics properties of matter in all these topics you are trained to solve a large number of problems and some of them are quite intricate this course is not exactly like that because the mathematics that we are going to use is quite Elementary in fact much more Elementary than what you would use in let us say

00:02:10 electromagnetism or mechanics or gravitation for that matter rotational motion especially but on the other hand the concepts that we discuss are very very deep they are radical so radical that at the beginning of the 20th century last century when physicist like Herz Einstein milikan name goes on and on Laurence bore Heisenberg when these people encountered they were completely startled so it doesn't mean that the subject is only conceptually difficult and mathematically easy it is thus that this particular

00:02:50 course introduces to you the conceptual basis in a very simple way but of course after that when you join for your graduation or postgraduation in physics you will start learning the mathematical aspects the quantitative aspects which actually is extraordinarily sophisticated the other thing about these topics that we are going to study is that there are some fundamental issues on which great physicists even ponder about for example wave particle duality what exactly does it mean what exactly is the meaning of the

00:03:28 uncertainty principle what does one need by what does one mean by measurement these are the questions that we don't normally ponder about when we study classical mechanics you were study you were taught particle properties you were taught wave properties you did not have to worry but here you do have to worry because the same physical system can show or exhibit both particle nature and wave like nature depending on the condition so there are lot of implications not only for physics but also for metaphysics actually of course

00:04:04 we are not going to get into that but the point that I'm going to make here or I'm trying to make here is that please don't take this lightly in a certain sense you can relax because there will not be too much of mathematics but in another sense you have to be completely alert because what we are going to discuss is really one of the most astounding achievements of humanity we can very safely say that and proudly say that so with this brief introduction it is actually good to announce our Moto what is the Moto

00:04:46 that we want to announce we want to keep it simple but we do not want to make it trivial and this was very beautifully expressed by none other than the great Einstein and you see that here in the next slide so it is good to announce what our Moto is at this particular point and what I'm trying to say is that we are going to try to make things easy but not trivial and this was very beautifully put by none other than the great Einstein which you will see in the next slide and if you look at the slide he

00:05:22 said simplify as much as possible but never oversimplify oversimplification gives your feeling that you have understood something or sometimes even everything when actually it is not the case and we shall try to follow this dictum so as I told you this slide displays for you what are all the topics that I'm going to cover so let me repeat so that it sorts of gets fixed in your mind the first topic that we are going to cover is photo electric effect and I'm going to spend a lot of time discussing this effect because not only

00:06:00 do we have to write down the famous Einstein formula for the stopping potential or the ionization potential we also have to describe the great experiments of herds and Mulan very very carefully and also Lenard I'm going to spend a lot of time on that and you should remember that Einstein got his Nobel Prize not for either his special theory of relativity or general theory of relativity but for photoelectric effect so when you read photo Electric effect in your cbsc textbook or any other textbook you may

00:06:33 Wonder there is nothing much to do why is it that he was given a Nobel Prize the answer to that came from Einstein himself he said that creating special theory of relativity was a child's play comparing to obtaining an explanation a proper description of the experimental results on photoelectric effect because when it comes to relativity he had the wisdom of 300 years of electrom magnetic Theory luren Spar and so forth whereas when it came to photo electric effect he was charting his own course so that is

00:07:08 something that we have to remember and therefore I'm going to discuss the experiments very very carefully and in great detail so when I speak of photoelectric effect I'm discussing the partical nature of light light is an electromagnetic wave you people have solved many many problems conversely it was observed by D broy where when he became aware of B's work on quantization that we can also ascribe a wave like nature to the particles so in the first case what was behaving like a wave classically started

00:07:42 behaving like a particle in some context in the second case what was behaving like a particle in your cathod tube or whatever when it goes to the atomic scale it starts behaving like a wave it start showing wave like property and of course a brilliant experimental verification came from the work of division and GMA and we are going to discuss that so these two are two complimentary aspects of the same what we call as the quantum mechanical systems and in the classical limit one of them exhibits only wav like nature

00:08:14 and another exhibits only particle like nature but in the quantum limit well it depends on the situation that is something that we are going to spend quite some time about next comes the nature of the atom this of course is an extraordinarily important topic for us to study because ever since the early dawn of intellect in humanity people always wondered about what the ultimate constituents of matter can be and of course there were many many theories for example in our own country there is this school called the

00:08:50 vishika school right they argued that everything is ultimately made up of atoms their propounder was a philosopher called Canada in a similar manner there was a corresponding School in the Greek civilization where democratus said that everything is ultimately made up of atoms of course there were counter theories where people believed that matter is actually continuous we don't really need the atomic nature and this discussion went on and on and on for example when you study electromagnetic

00:09:26 Theory or let us say problems in moment of enery or problems involving rotational motion rigid bodies you assume that the density is a continuous function of space time coordinates okay if it is rigid it is a function of only space so you treat them as continuous functions you treat the charge density as a continuous function whereas if you want to understand thermodynamics you would have to employ the molecular hypothesis so you know sometimes it is convenient to treat it as a Continuum sometimes you actually

00:09:58 need the molecular h hypothesis to understand equilibrium phenomena so this is a fundamental problem to be actually decided by experiments and their interpretation and when it comes to atoms the primary unit of chemistry let us say the most important information the most important Insight came from raford scattering and who gave the planetary model of the atom we are then going to argue that planetary model of the atom gives rise to to whole lot of problems what are the problems that it is going to give rise

00:10:34 to well there is a problem of stability there is a problem of why my electron will not fall off into the proton so on and so forth and in order to understand that bore proposed actually his famous bore model where he was able to quantize angular momentum quantize energy levels and he was able to describe Atomic systems so that is something that b did this is something that we will be studying at a great detail so what I want to do at this particular juncture is to take a few minutes and uh recall to your memory

00:11:12 whatever you have studied and point out what are the differences that we are going to see when we study the so-called Atomic phenomena the microscopic phenomena Vis whatever you have studied in your microscopic phenomena be it electrodynamics or be it the classical mechanics so let us start with a few preliminaries and it doesn't harm us to get an overview by the way I'm not going to start the lectures today I'm only going to give you this kind of a broad overview and an introduction the real

00:11:46 course will start from the next lecture when I will start discussing the great experiments on photoelectric effect okay so let us start with the Newtonian mechanics because you must have heard of the word quantum mechanics and we want to see what the differences are I am not in a position to tell you what all the differences are I'm going to mention some of them and even what I mention falls into the realm of the so-called old quantum theory they get much more sophisticated and refined later so please don't think that

00:12:25 whatever I have told you is the gospel truth there is an element of Truth truth but there is also some amount of ambiguity in it that is fine because all of learning is learning unlearning and relearning that is the process that we go so what we are saying is that in Newtonian mechanics suppose you give me a particle let us say of mass m and it has an initial velocity V KN and an initial position or not and you give me a force this force can depend on on where the particle is located and the time so you can imagine

00:13:05 for example an electric field produced between two capacitor plates which are getting charged or discharged then to a large extent my electric field between the plates is uniform but it is a function of time actually if you take the edge effects into account it will also be a function of the position let us do that so what does Newton tell us Newton tells us that if you give me the initial position and if you give me the initial momentum MV KN and if you give me the force then rest of it is a matter of detail what

00:13:40 does detail mean I can tell you the position and momentum at all later times all observables that we construct are actually functions of position and momentum or maybe their derivatives if you really want to look at something like a torque or whatever therefore Newtonian world is what we call as deterministic everything is determined so what do you do if you consider the simplest example of one dimensional case I will write m d^2 x by DT ^ 2 is equal to let us say f of x comma T then I integrate you people have

00:14:27 integrated in fact if my force is independent of time that job becomes even easier I will write m d by DT this is the general method is equal to F ofx and what do I do on the right hand side I will write it as M DV by DX DX by DT is equal to F so therefore that means I can write my this equation and I can start integrating this equation so how am I going to integrate this equation so I can write m v DV is equal to F DX that is how I'm going to write I can integrate this remember my f is a function of X so

00:15:20 let I let us say I integrate it from V not to V so I'll put a prime here and I'll put an X not to X Prime X and put an X Prime here the right hand side is an integral which you can evaluate you have been taught in the calculus so for notation sake I will call X comma X KN that is a standard method of calculus and this is going to give me m v^ 2 by 2 v^ 2 2 is equal to I of x x now the next step is even simpler I will transfer this to the right hand side and then I will get an expression

00:15:59 for y DV by DT and I will integrate the right hand side with respect to time actually I can show you that step that is the procceed that we are going to employ so let us say for Simplicity sake V is equal to 0 then what am I going to get I'm going to get V ^2 is equal to 2 by m i into x x not otherwise I'll have to write the other term then therefore my V is given by square root of 2 by m i of X comma X and this is nothing but DX by DT so we employ the same procedure and we write dx/ otk 2 by m into I is equal to DT

00:16:52 integrate the left hand side integrate the right hand side you get a function of X you invert it then you can write X as a function of time in fact you can repeat this for your harmonic oscillator problem for instance and you will immediately find out what the solution is so in other words Newtonian mechanics is completely deterministic number one number two if I go back to the original slide and if you look at m v and R not there is no restriction on what your velocity can be and what your position can be there is

00:17:26 absolutely no restriction that means you can specify your initial condition in any way you want with any degree of precision and then Newtonian mechanics will take over and it will give you the position velocity acceleration angular momentum whatever you want at all later times and therefore what we have is a completely deterministic system and if you have good mathematical skills and if you have a powerful computer and you know how to write down your program then there is no problem at all what happens is that you

00:18:00 should be able to solve everything that is how people were able to work out for example the planetary Dynamics perturbation because of various planets so on and so forth now just to give you a flavor of how quantum mechanics is going to be different so as I told you this is some kind of a general overall view let us take the example let of gravitational force so what do you have your f is given by g m m by R squ that is what you have and let us say that your body is sitting in a circular orbit these are the problems to solve the

00:18:39 earth going around the Sun in a circular orbit a satellite going around the Earth in a circular orbit moon is in a roughly circular orbit these are the problems geostationary orbits if you want to do that you will write mv^ 2 by R that is what you are going to write of course this Mass goes away and when you study your gravitational phenomena you will study the the importance of cancelling Mass on both the sides in a great detail this R goes away and you get a beautiful expression V ^2 is equal to GM by

00:19:15 R now you see there is a free parameter R here if you give me R then v² get fixed and if the v² gets fixed then of course its kinetic energy is fixed but the important point is is that this R is a continuous parameter and you can change it at will so what I would recommend to you people all of you students used to go back open up your YouTube and look at how India was able to send its probe to Mars the famous mongan so what did they do they first launched it into an orbit very close to the Earth then there were these

00:19:58 sling shots is that right there were the sling shots which kept on changing the orbit that means when you are changing the orbit this R is changing of course there are other things that change because uh there is also a question of angular momentum because in general the orbit is elliptical but they were able to change it continuously so if I am to show it to you in a schematic manner you first put it in this orbit then give it a shot then it goes like this okay forget about this in this septical orbit

00:20:30 then you get it another shot like this then it enters an even larger orbit then you give it another shot somewhere here let us say then it escapes and it gets into the orbit of Mars and it goes and here where you want to put your satellite this is my Earth and this is satellite right depend it's entirely under your control so this is like a caricature of Mangan so look at that and they did it in a very very controlled way because every obeyed Newtonian mechanics but when you study bore atom bore will say

00:21:03 no no no you cannot simply write the equation G mm by r² is equal to mv^ 2 by R because this R cannot be arbitrary is that okay this R cannot be arbitrary it should satisfy a very special condition so what does that condition mean it says that for example if a particle can be in this orbit a particle can be in this orbit but a particle cannot be in any of these orbits inside or any of the orbits between these two and that is what is called as the quantization condition that is why it is called as quantum

00:21:39 mechanics and all of you are familiar with the condition M VN RN is NH bar that is what you're going to study right this is the orbital angular momentum this orbital angular momentum cannot take any value that you can give we should satisfy the condition that it is an integral multiple of H bar H bar is your famous planks constant H by 2 pi H is usually called the planks constant and H by 2 by is your H bar and this plank constant actually came from the photo electric effect and that is something that you are going to study in

00:22:16 GED detail depending on the experimental thing however in order to understand photoelectric effect there are some things that we have to recall and let me spend some time telling you what is it that you have to recall so remember you studied electrostatics magnetism and of course you study electromagnetism I want to recall to your memory the Salient features the most important features so what is it that we learned when it came to electricity and magnetism that is very very important so all of you have solved

00:22:59 for example the problem of RC circuit and you have studied how energy flows into the electric field between two capacitors so when you work out the energy you write an beautiful equation Epsilon 2 by e^ S is the energy density of the due to the electric field so electric field is there that is the most important role of a capacitor in modern appliances it puts charge and it stores energy and you can use that energy any time you want and where does that get the energy from of course it gets the energy from the battery or the

00:23:51 cell to which you have connected so let us not forget that it is not coming from nowhere so you have your resistance you have your capacitor and then you have your charging thing and then you put a switch the minute to connect your switch the current flows the positive charges accumulate and now the energy gets stored in between them so electric field can store energy in a similar manner when we look at induction for example there is a corresponding magnetic energy which can be again harnessed and

00:24:24 that you write as I hope I'm writing the correct expression B squ this is the magnetic energy density you people have studied this so therefore energy can be stored either as magnetic energy or as electrical energy and it can be used but then we have another law the Great Law Faraday law of induction and what did teach us he taught us that time dependent electric field implies non- Vanishing magnetic field so how do I write that so the change in the magnetic flux or the induced EMF is nothing

00:25:22 but inte the CH minus Delta 5 by delta T which is the the electric flux okay time dependent electric field can actually produce a magnetic field that is what we are going to write and this is the magnetic flux okay and again you are familiar with that problem because if you start producing a Time dependent field then immediately you make use of this equation and find out how much energy is stored in the magnetic field but even more importantly what farad taught us was that since a Time dependent electric

00:26:07 field can produce a magnetic field and in a similar manner Maxwell taught us a Time dependent magnetic field can produce an electric field what happens my electric field energy can go to magnetic field energy which I will rote it as em and visce Versa and the great Insight that came from this because of Maxwell was electromagnetic waves electromagnetic waves where the electric field acts as a source for the magnetic field and the magnetic field acts as a source for the electric field that is what Maxwell said

00:26:46 and when you work out you find that these electromagnetic waves travel with a speed of one over root Epsilon mu so all of you know what the values of Epsilon mu R mu is 4 4 Pi into 10^ ofus 7 Newton M or whatever serly Epsilon but the most important thing is when you substitute these values you will get this great Magic Number 3 into 10^ of 8 m/s that is what you're going to get independent measurements of the speed of light by lot of people including gromer who actually determined it by looking at the eclipses of Jupiter

00:27:25 shows that light also travels with the same speed so since it is extremely unlikely that there can be two important quantities with the dimension of light namely the speed of light itself and 1/ root Epsilon mu Maxwell made the great conjecture that what we call light is nothing but one part of the electromagnetic wave spectrum that is what he said and indeed that was very very brilliantly verified by hers and in our own country by the great man JC BOS who was able to produce microwaves when he was working in Presidency College and

00:28:16 he was able to see all of them all in fact he was able to see defraction phenomena in interference in the microwave region for the first time and today his portrait addons the famous Hall of f Fame you know the Great Hall of Fame of electrical engineers so we can be very legitimately proud of that that is what we have now I am telling you all these things because I want to lay the foundations for the importance of the photoelectric effect and you people in your class 12 again when you are preparing for your exam or when you

00:28:47 are studying in your class you know how to write down the expression for the electric field and the magnetic field so let us do that so the most important concept for you is that of a plane wave is that right so what do I do I say a particle has a frequency Omega this is of course the angular frequency and has a wave vector k we know what it is right so 2 pi by mod K is the wavelength that is what I have and K divided by mod k is the direction of propagation right and Omega is 2 pi new where new is your

00:29:49 frequency so what do we have omega equal to CK or if you feel like new Lambda equal to C where C is 3 into 10 ^ of 8 m/s that is what you have studied so once I establish this I can write down my electric field and what is the expression for my electric field well let me write it down for a plain polarization so I will write e not and let me assume that the propagation direction is along the Z axis so I will write e i cos Omega T minus K set this is something I want you to pay a great attention to and all of you know

00:30:34 how to immediately write down the expression for the magnetic field we are obviously employing the SI units my magnetic field will be B KN by C into J cos Omega T minus K that is what you're going to get so what are we going to do now what we are going to do now is to ask if this electromagnetic wave is propagating which is nothing but light at a certain frequency Omega and a certain wave number k 2 pi by K is of course my Lambda where is the energy after all we are all the time speaking of solar

00:31:18 energy nowadays because it is clean energy in fact this cleanest form of energy was first employed by Nature when photosynthesis was started by the plants and we all live because of that because of photosynthesis not only is energy harnessed but we also get a lot of oxygen so you see my energy is stored both in the electric field and the magnetic field and I can write down my Expression e^ 2 plus b² I suppose there is some Epsilon KN here and there is a b mu here Epsilon 2 1 / 2 mu not or whatever is that okay this is my energy

00:31:54 density this is where the energy stored is and what is the important point that we have to notice here the important point that we have to notice here is that my wave is oscillating so when you speak of an oscillation you give me the amplitude you give me the frequency and you give me the propagation Direction and what this expression is telling me is that the energy carried by the wave is completely independent of what the frequency it is a function entirely of the amplitude that is what it is saying

00:32:26 in other words my Omega r k they do not contribute they do not contribute to energy that is the most important thing that is also true of the oscillators therefore in fact this can be looked upon as a collection of oscillators in other words my Omega or k only tells me how many times it is oscillating but the real energy is actually in the amplitude how many times you go from you know minus X to plus X or whatever that is the information that is being given to you so it depends only on E and B and

00:33:04 this is something which is well known because as you keep on increasing the intensity of light for example you have 50 watt bulb 100 watt bulb 500 watt bulb then you are getting more and more energy if you want to heat you speak of rating of 1.3 kilowatt or whatever whatever is that okay that is what happens so this is a very well established fact and everything seems to be fine however what physicists observed was that this is all right if you are going to study electromagnetic Theory only in isolation but this gets into

00:33:36 trouble when you look at what when you are going to look at electromagnetic Theory so let me rate it it fully electromagnetic Theory plus thermodynamics I cannot study anything any fundamental the in isolation it has to be studied together with thermodynamics because all real systems are at some finite temperature and all systems are interacting with their environment that is an extraordinarily important thing how did you produce light you stck a match and you light at the lit the candle or you are going to

00:34:17 heat something for example you may light a set firewood on fire or you may have Embers burning coals or whatever all of them their laws are governed by thermodynamic and there are very very important principles in thermodynamics for example the first law tells you that the total energy should be conserved which is also there but equally important which you study in your kinetic theory of gases is that at any temperature at any temperature each mode this is very very important what is a mode degree of freedom

00:35:02 carries an energy 3x2 KBT KB is your famous boltzman constant right it carries that unit of energy in other words thermodynamics asserts and there is extraordinarily good experimental verification in fact we may keep on changing our laws of physics but thermod damic is going to be robust is that okay that this equip partition is a very very robust and a well established result there is a very good experimental evidence and this depends on the number of degrees of freedom that is a very important thing and at any given

00:35:43 temperature only at t equal to Z can your energy be equal to zero the minute to switch on the temperature there will be oscillations you will start interacting with whatever is there and it will also acquire an energy and each mode will get that you students are thoroughly familiar with this because you have studied monoatomic gas diatomic gas and you know how to take for example the number of degrees of freedom into account for example if you have a diatomic gas should you worry only about the vibrational States or should you

00:36:20 worry about the rotational States so on and so forth for example for a monoatomic gas so I should uh make a correction here I'm very sorry about that let me come back here at any temperature each mode Cates not 3x 2° but half KBT so not 3x2 but half KBT units of energy that is what it is so if I come back to monatomic gas gases are sitting in three dimensions so that will be 3 into half KBT that is the energy carried by each molecule that is the basis of the kinetic theory of gases and if you have

00:36:57 n molecules you will multiply it by the n and this will be the total energy of the system and starting with this you people know how to obtain you know the famous gas loss like PV equal to RT so on and so forth by applying pressure either expand gas expands or whether the gas compresses so on and so forth that is what we have so you may say so what what does that have to do with my electromagnetic waves so what we shall do and this is the great experiment that we are going to look at imagine that you

00:37:25 are going to confine your radiation in a pipe it is closed here and this is at some temperature this cavity this is cavity is a temperature and there is electromagnetic wave so it gets reflected at this point so the wave is getting confined you know that my K cannot be continuous so it will be something like 2 N Pi so K will be quantied K will take discrete values so K will be essentially K will be proportional to n by L where L is the length of the cavity how do you get this length of the cavity so you create a standing wave and

00:38:13 you get that now you see my K has become discrete in fact if you want a wave there is a minimum KL which is given by one o l it gets multiplied by 2 pi or whatever you don't have to worry about that so then you will have K1 K2 K3 etc etc and all these are discrete modes these are the degrees of freedom so even you fix the in even if you fix the intensity there is nothing in physics which will tell me whether I should look at K1 whether I should look at K2 or K3 all these degrees of freedom are there

00:38:44 and how many are there well I can draw a picture for you so that is very much like a string that is what I have this is my fundamental mode the so-called first harmonic this corresponds to K1 then I will write this there is one node then I will write this there is a second node so on and so forth there is no limit to the number of nodes that means knen can take arbitrarily large values so what are the what is the statement that we are making we are saying that if you confine electromagnetic wave for example within

00:39:26 a certain region Say by putting reflectors or whatever let us call it as a cavity this is also very much like a vibrating string if you feel like then it can take arbitrary large values means it has infinite degrees of freedom it has an infinite number of degrees of freedom an infinite number of degrees of Freedom now what do I do I couple my electromagnetic Theory with thermodynamics electromagnetic Theory plus thermodynamics so what does electromagnetic Theory tell me it tells me my energy density is proportional

00:40:20 to mod e^ s and this tells me it is proportional to number of M degrees of freedom into half KT and at for all finite temperature however small T may be so long as it is not equal to zero this is infinite and this is finite so as students you people would have played a game where you know you suddenly get finite number is equal to infinite number and immediately your friend tells you or if your friend is committing a mistake you your friend that you're doing something something illegal you looking at something like 0

00:41:00 by0 which is not well defined and that is why you are getting that you have no business to do that because 0 by0 is not defined in a similar manner here the problem that we are getting is not because of mathematical problems but there is a deep physical problem this tells me there must be a finite energy density this tells me there is an infinite energy density and people actually performed very very careful experiments and what is it that they found what they found was that actually the predictions depending on energy

00:41:30 depending on the E squ or the predictions depending on this half KT was not going to work out the way we are looking at it what it required was quantization that is you don't associate energy with the amplitude but you associate energy with the frequency of course you also associated with the amplitude in a very sophisticated way and I'll come to that later when I discuss Einstein's explanation for the photoelectric effect but you see when I'm speaking of photo electric effect I

00:42:04 am not thinking of a small correction it is not mean these are all great achievements for example the discovery of the planet Uranus is a great achievement but here we are looking at a deep contradiction in physics deep Paradox deep problem in physics and this is a kind of thing which gets shown up which actually gets displayed when we look at a phenomenon like Photo Electric effect or a Compton scattering or black body radiation therefore what Einstein's photoelectric effect did was to actually

00:42:35 resolve this particular problem that is something that we have to know and the beauty of this is that once it was resolved this idea of plank constant you know plank gave that constant for in order to explain this Einstein used it very intelligently to understand photoelectric effect and later bore put it into even greater use and a fantastic use by applying it to the atom and they actually resolved the problem of the so-called stability when I'm using the word that he resolve the problem that of course is

00:43:08 in a very very qualitative way you would have to study quantum mechanics at Great depth and in great detail is it okay but still you should be able to actually understand at least qualitatively whatever is happening so if I come back to my slide whatever I'm showing you I'm going to discuss after the photoelectric effect raford scattering planetary model of the atom and the bore model is that okay where you have quantization of angular momentum quantization of energy and atomic transition so on and so forth

00:43:40 and after that we are going to look at the interior of the atom so that is the beauty initially raford bombarded the gold foil with alpha particles and he showed that most of the atom is empty a similar experiment was done by host with electron being and he showed that they can actually resolve the structure of the nucleus it is of the order of the size it size is of the order of 10 the^ of-5 m that is what we are going to do and there we are going to study the properties of the atomic nucleus protons

00:44:13 neutrons why are some nuclei stable some are why are some nuclei unstable we are going to study the famous alpab beta gamma DC fusion and fusion after studying Fusion I'm going to give you a idea actually of how we can understand the fact that sun has been able to produce such an enormous energy for the billions of years and it will continue to live for few billion years we are going to discuss that and then of course there is an important part of nuclear fishion and fusion reactors I'm not

00:44:47 going to spend too much time on that because there is not much to discuss except a matter of detail I may just hinted that and that should be essentially the course which is contained between chapter 11 and so we are not going to hurry up we are going to take our time and we are going to study them for those of you who feel that okay this is all there in some exotic nature somewhere deep subatomic physics why do I need quantum mechanics at all what is the role of you know planks constant or black body radiation

00:45:16 or photo electric effect for everyday life you should understand that impact of quantum physics actually transcends everything is that okay it is used everywhere today and and our later latter part of the 20th century and the beginning of 21st century would be nowhere without quantum physics so let me remember in your future chapter ON Semiconductor devices that you are going to study is that okay all the phenomen based actually on quantum mechanics classical mechanics cannot explain and you know all of modern technology is

00:45:50 actually based on semiconductors and its various avatars that is what we have so I have listed some of them to you in this slide your laptops and computers smartphones music recording system home appliances or for that matter in medicine your You Know M magnetic resonance amazing pet scat PET CT scans etc etc all of them are based on developments in quantum physics in other words today we live in a world where quantum physics not only deepens our understanding into the microscopic microscopic world or the microcosmic

00:46:24 world you know of atoms nuclear and Elementary particles it also helps us to produce better and better instruments to make our life easier is that okay so that is something that we have to know within physics itself it covers various phenomena in particle physics in nuclear physics in atomic and molecular physics in condensed matter physics and there is something even called as Quantum thermodynamics or Quantum statistical mechanics it tells you how to understand cosmology what the early Universe was

00:46:56 what will be the fate of the universe what was the beginning of the universe is that okay all these are intricately tied in fact completely tied to our understanding of quantum mechanics and the beginning of quantum mechanics is something that we owe to what Plank and Einstein did historically although plank introduced the concept of plank constant he did not believe in the existence of photon the real impetus you know the real belief in the concept of the photon was actually from Einstein who belied

00:47:29 that it's really existing it is not a mathematical construction and many people thought that he was actually not being very sensible when he made that statement is that okay you will get a glimpse of that when I discuss photo electric effect later but that is how it is is that okay so that means all the developments that have taken place in the 20th century and taking in place in the 21st century wo a lot to PL and Einstein of course Einstein also Al gave us special relativity and general relativity his three great papers were

00:48:01 all published in 1905 that's why it is called analist miraculous year of Miracles as far as physics are concerned is that okay he wrote special relativity he wrote a paper on Photo electric effect and he wrote a paper on bronan motion which established the molecular hypothesis of boltzman so there were fundamental papers and you are going to study them slowly as advance in your career in physics but then if at all we have to begin we have to start with photoelectric effect and that is what we

00:48:33 are going to do so as I told you it tells us about the physics of the interior of Stars For example helmold made a calculation and said that lifetime of the sun is not more than 21 million years and it cannot live for more than another 5,000 years whereas we know that sun has been there for at least 4.6 billion years okay few million years ago there were actually dinosaurs and things like that and AG of the Earth was a big problem for example Kelvin said that Earth cannot be more than 100 million years old helmold said the sun

00:49:10 cannot be more than 21 million years old how can Earth be older than the Sun that is first contradiction and more importantly we know that Earth has been there for at least 4.5 billion years old how do I know that I know that because of the evidence coming from from the fossils and rocks and so on and so forth all these issues get resolved about the you know physics of the planetary system physics of the earth once we understand Quantum phenomena and radioactivity fusion and fion that is same thing that

00:49:44 you should know therefore in other words our scale is enormous starting from let us say 10 the^ of minus5 M to probably 10 the^ of + 15 or even more let me stop there there are 30 orders of magnet ude there is no Theory whose scope is as large as that as deep as that and that is something that we are going to study so to summarize whatever I have told you we have studied Laws of Motion we have studied thermodynamics we have studied properties of matter we have studied waves and oscillations and we have actually

00:50:22 studied that electrostatics and energy in electromagnetic field magnetostatics and energy stored in magnetic field I'm summarizing whatever I worked out for you induction displacement current electromagnetic waves we are going to make use of them is that okay and once you brush all those things read farad law of induction see how energy is stored in capacitors how energy can be stored in inductors look at them there is a beautiful analogy you know between mass spring constant etc etc of an oscillator and capacitance and inductor

00:50:55 and resistance of in electrical circuit resistance is like a frictional force damping force was that okay when you do that if you come back from the next lecture onwards lecture two we will start our photo electric effect with basic preliminaries so at this point I'm going to stop so although it might appear to be some kind of a story actually it is more than a story it is more than history because I want you to go back open your 11th and 12th standard books read your chapters on gravitation

00:51:25 electricity magnetism mechanics and thermodynamics and Optics where you assumed you know that light is a ray you did not even use the fact that it was a wave is that okay me assimilate all that remember that and you come then you will see how radical photoelectric effect how radical a theory that photoelectric effect gave rise to and that we will take up in the next lecture okay goodbye